Method and apparatus for cooling hot gases

ABSTRACT

A method and apparatus for cooling the exhaust gases from a molten phase furnace, such as a smelting furnace. The exhaust gases are conveyed from the furnace to a vertically extending cooling shaft disposed above the furnace, where the exhaust gases are indirectly cooled by means of a cooling gas in a circulating system. From the cooling shaft, the exhaust gases are conveyed to a waste heat boiler, where heat is recovered from the exhaust gases as saturated or superheated, pressurized steam.

The present invention relates to a method and apparatus for cooling theexhaust gases from a molten phase furnace, such as a smelting furnace.The exhaust gases are conducted from the furnace via a vertical coolingshaft into a waste heat boiler, where heat is recovered from the gaseseither as saturated or superheated, pressurized steam. The steam isutilized for electricity generation.

The present invention is especially suitable for cooling of exhaustgases from smelteries, for example, from the melting processes of metalsulphides. It is also applicable to other processes in which hot, dirtygases have to be cooled and in which water-cooled surfaces mayconstitute a risk.

The exhaust gases from metal smelteries are typically hot gases of1,100° to 1,400° C. containing solid particles, dust partly in a moltenform, and gas components which condense to a solid phase when cooled toa temperature of, for example, 200° to 400° C.

To meet the environmental requirements, such gases usually have to becooled to a sufficiently low temperature prior to further treatment. Assmelteries use sulphide as a raw material and the sulphur containedtherein is transferred to the gas phase as sulphur dioxide (SO₂) in theoxidation stage after melting, the SO₂ content of the gases normallyrises significantly, to a level of 7 to 15% or even higher if air isreplaced by oxygen in the melting stage.

A conventional gas treatment of such a process comprises the steps of

first cooling the gas in a waste heat boiler which generates saturated,sometimes superheated steam and

then, after the boiler, separating solids from the gas, for example, byan electrofilter. The gas cleaned of solids is then conveyed to asulphuric acid plant in which SO₂ contained in the gas is used as a rawmaterial. The steam boiler is used because it facilitates electricitygeneration for the smeltery by means of a steam turbine. Usually,electricity is generated in excess and the surplus is sold.

Most smelting processes of metal sulphides employ a smelting furnacestructure in which the easiest and simplest way of discharging theexhaust gases is to lead them upwards and to discharge them through anopening in the furnace roof. For example, Finnish patent specification65632 and U.S. Pat. No. 4,087,274 propose smelting furnaces where theexhaust gases are discharged through an opening in the furnace roof.

However, this arrangement involves a risk if the steam boiler and thefirst heating surfaces thereof are disposed above the smelting furnace,extending straight upwards form the opening in the furnace roof.Bursting of a boiler tube in such a structure and the leak of watercaused thereby would expose the smelting furnace to a danger ofexplosion, as water spraying from the leaking point automatically comesinto contact with the molten material in the furnace.

A boiler arranged above a furnace could be provided with a superheater,i.e. with steam not water cooled heating surfaces. In that case, theportion above the furnace would constitute a superheater and thedangerous evaporating surfaces containing boiler water could be locatedfarther off. However, this is impossible in practice for the followingreasons:

one of the biggest problems encountered in cooling the gases is thatdust particles stick to the heating surfaces, whereby the surfaces tendto become clogged and the heat transfer decreases remarkably. A rise inthe surface temperature makes the phenomenon still worse. Therefore, theheating surfaces in such boilers are usually constructed to give an ashigh cooling effect as possible while generating saturated steam. Theyare not constructed as hot superheater surfaces. If necessary, the steamgenerated in these boilers is superheated in a separate superheatingboiler arranged in front of the steam turbine.

at steam pressures in question (below 100 bar), the thermal energy forsuperheating is too low, when compared with the thermal energy forevaporation, to provide a cooling effect by superheaters only in thatportion of the boiler which is arranged immediately above the furnace. Asteam pressure exceeding 100 bar would, on the other hand, result in thetemperature of the evaporating surfaces rising too high in view ofcleaning.

A conventional boiler arrangement used in these smelteries is ahorizontal boiler arranged at a side of the smelting furnace, therebyavoiding the risk of explosion caused by water leaks. A similar boilerarrangement is used, e.g., in a smelting process disclosed in U.S. Pat.No. 4,073,645. The arrangement has proved to operate well, but theboiler structure is expensive and space consuming, and the total effectof the arrangement thereby remarkably increases the price for gastreatment.

An object of the present invention is to provide an improved method andapparatus in comparison with those described hereinabove for cooling theexhaust gases from smelting or combustion furnaces, and especially toprovide an arrangement which is safe in operation.

Another object of the invention is to provide a simple apparatusconsuming as little space as possible for cooling of exhaust gases.

A further object of the invention is to provide an economic method forheat recovery from the exhaust gases, in which method the heat of thehot gases may be optimally utilized and the temperature of the exhaustgases be lowered to a level required for gas cleaning.

A still further object of the invention is to provide an arrangementwhich both improves the safety in operation and ensures the electricityself-sufficiency of the smeltery or substantially contributes thereto.

The method according to the invention for providing the objects of theinvention is characterized in that exhaust gases are cooled in thecooling shaft by cooling said cooling shaft with gas. Cooling ispreferably effected by means of gas circulation using uncondensable gas,such as air or nitrogen. The heat transferred from the exhaust gas tothe cooling gas during the cooling stage may be employed in preheatingthe boiler water in the waste heat boiler and in heating and/orevaporating the condensated steam in the steam circulation. According tothe invention, exhaust gases are cooled in two stages and by means oftwo different heat transfer mediums. In the first stage, exhaust gasesare cooled in the cooling shaft where gas is used as a cooling medium.In the second stage, heat is recovered in the waste heat boiler by usingwater and water vapor as a heat transfer medium.

The apparatus according to the invention is characterized in that heattransfer surfaces are disposed in the cooling shaft for indirect coolingof the shaft by means of gas. The cooling shaft is preferably incommunication with a cooling gas circulation system, which comprises

heating surfaces in the cooling shaft for transferring heat from theexhaust gas to the cooling gas,

a heat exchanger for cooling the cooling gas,

tubes for transferring the cooling gas from the cooling shaft to theheat exchanger,

tubes for transferring the cooling gas from the heat exchanger to thecooling shaft, and

a circulation fan for circulating the cooling gas in the gas circulationsystem.

The cooling shaft in accordance with the invention may be arrangeddirectly above the furnace in alignment with the opening the furnaceroof. In such case, the exhaust gases rise upwardly in the furnace anddirectly enter the cooling shaft. The waste heat boiler is preferablydisposed next to the shaft and the furnace. In the cooling shaft, theheating surfaces are arranged so as to effect the heat transfer in theform of radiation heat transfer. The shaft walls may be composed e.g. ofheat transfer surfaces wherein gas flows. The waste heat boiler isprovided with convection heat transfer surfaces.

The boiler arrangement according to the invention comprises twosections, wherein a vertical, shaft type section is disposed above thefurnace, for cooling the gases to a temperature range of 600° to 900° C.An optimal temperature depends on the process and the smeltery and,exceptionally, it may even be outside the above-mentioned temperaturerange. After-cooling of gases, normally to a temperature range of 330°to 380° C., takes place in a boiler section arranged next to thevertical section and communicating therewith, at the side of thefurnace. The boiler section is primarily provided with convective heattransfer surfaces. In the vertical shaft, heat transfer is primarilybased on radiation.

In the arrangement according to the invention, only the latter section,i.e. the waste heat boiler, is constructed for the generation ofsaturated or slightly superheated, pressurized steam. The pressure ofthe steam generated is typically 25 to 80 bar.

The shaft section is cooled by means of pressurized, uncondensed gas,inert with respect to the process, such as air or nitrogen. Thetemperature range of the gas in the cooler is adjusted to be suitablefor the temperature of the heating surface so as to minimize fouling ofthe heating surfaces. The temperature of the surface in contact with thegas to be cooled depends on the process conditions. When, for example,gas with a high SO₂ content (10 to 15%) is cooled, the temperature ispreferably 250° to 320° C.

Cooling of the exhaust gas in the shaft is brought about by the coolingsystem formed by the cooling gas circulation system. The cooling systemcomprises

a heating surface of the shaft

a cooler/coolers for the circulating gas

a circulation fan

a pressure compressor, and

circulation tubes.

As no phase alteration is involved in the gas cooling, the mass flows ofthe gas circulation normally become large in volume. If the gas isunpressurized, the circulating volume flow is very big. It may bereduced adequately by pressurization, whereby the power consumption ofthe circulation gas fan remains at a reasonable level.

Another advantage, even possible necessity, gained by pressurization isthat the heat transfer resistance of the heating surface of the shaft onthe gas circulation side is sufficiently lowered. Heat transfer isremarkably improved by pressurizing the gas, and the temperature of theheat transfer surface approaches the temperature of the circulation gas.In this manner, the temperature of the heating surfaces in the shaft iscontrollable. This is very important because strong radiation,prevailing in the shaft is capable of raising the surface temperature toa harmful level in spite of the scaling phenomenon unless the surface issufficiently cooled. An adequate pressure level is >15 bar, preferably15 to 25 bar.

Unless utilized, the heat transferred to the gas circulation is wasted.The heat is preferably utilized by heating both the boiler feed waterfor the steam circulation and the cold condensate discharged from theturbine condenser. The steam power of the boiler is thereby increasedand correspondingly the electricity generation. Whether an investment ina preheater is worthwhile, depends on the smeltery and, locally, itdepends on the electricity requirement and the electricity price.

An advantage of the arrangement of the invention resides in that a leakin the vertical section, i.e., the shaft, discharging gas into the shaftneither endangers the smelting conditions nor the safety of thepersonnel at the smeltery. Furthermore, the arrangement according to theinvention is easy to accomplish and its space requirement is relativelylow. At the same time, it also facilitates sufficient electricitygeneration from recovered heat.

When compared with direct cooling by cold gas, an advantage of indirectcooling of exhaust gas in accordance with the invention is that thearrangement of the invention brings about much smaller gas volumes,thereby benefiting the gas cleaning. Addition of gas to a hot exhaustgas flow may also be problematic; even keeping the gas nozzles open maybe difficult.

The solid material separated from the gas cooled by the method accordingto the invention may simply be returned to the smelting furnace with noneed for any special measures because neither the exhaust gas nor thesolid material has been treated directly with any substance which couldbe harmful when brought into contact with melt.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described in the following, by way of example,with reference to the accompanying drawings, in which

FIG. 1 is a schematic illustration of an arrangement for cooling exhaustgas in accordance with the invention, and

FIG. 2 and 3 are schematic illustrations of two other arrangements forcooling exhaust gas in accordance with the invention.

FIG. 1 shows a two-stage arrangement for cooling exhaust gases from asmelting furnace 10. In the first stage 12, exhaust gas is cooled by agas circulation system 14 and in the second stage 16, heat is recoveredfrom the exhaust gas in a steam boiler 18.

The roof 20 of the smelting furnace 10 is provided with an opening 22,wherethrough the first cooling stage in the smelting furnace is incommunication with a vertical cooling shaft 24. The exhaust gases flowvia the opening in the furnace roof into the shaft 24 and further to thesteam boiler 18 in the second stage.

The walls 26 of the shaft 24 are composed of heat transfer tubes 28,wherein pressurized cooling gas, such as air, nitrogen or other inert,uncondensed gas, flows. The heat transfer tubes form a radiation heatexchanger in the shaft.

The gas tubes 28 in the shaft are connected by circulation gas tubes 30and 32 to a heat exchanger 34, where the gas circulation, i.e. thecooling gas heated in the shaft, is cooled. Gas is circulated in thecirculation gas system by a circulation gas fan 36. A pressure of >15bar is maintained in the circulation gas system by means of a pressurecompressor 38. In the example, the circulation gas is heated, forexample, to about 300° C. in the shaft and is cooled, for example, toabout 220° C. in the heat exchanger.

In the example of FIG. 1, the heat recovered by cooling of the shaft isrecovered in the gas circulation system which is provided with one heatexchanger. The heat recovered in the shaft is not, in the example shownin FIG. 1, employed for electricity generation. Electricity is onlygenerated by the heat recovered in the steam boiler 18 in the secondheat recovery stage.

The cooling shaft 24 is connected to the steam boiler 18 by means of atube 40. In the steam boiler, heat is recovered from the exhaust gasesprimarily by convection heat transfer surfaces 42. In the example,saturated steam of 40 bar is conducted from the steam drum 44 of thesteam boiler into a steam turbine 46. A generator 48 connected to thesteam turbine generates electricity. The steam discharged from theturbine is condensed in a condenser 50 and conducted by means of acondense pump 52 into the feed water tank 54 of the boiler. From thefeed water tank, the feed water is returned by means of a feed waterpump 56, at a pressure of 40 bar and at a temperature of 105° C. to thesteam drum and further, by means of a boiler circulation pump 58, to theheat transfer tubes 42 of the boiler.

In the arrangement shown in FIG. 2, part of the heat recovered to thegas circulation system is utilized in the steam system. The elementscoinciding with those in FIG. 1 are identified by the same referencenumerals. Deviating from the arrangement of FIG. 1, the arrangement ofFIG. 2 provides that the boiler feed water is led from the feed watertank 54 by means of the feed water pump 56 into the tubes 30 of thecirculation gas system, and more specifically, into the preheater 60 ofthe feed water, said preheater being disposed in front of the heatexchanger 34. The feed water is heated to a temperature of 230° C. inthe preheater. The cold condensate discharged from the turbine condenser50 is also heated by utilizing the heat of the circulation gas system. Acondensate heater 62 is disposed between the feed water preheater 60 andthe heat exchanger in the circulation gas tubes 30. The heat exchanger34 takes care of the final cooling of the circulating gas. Thearrangement of FIG. 2 is capable of increasing the steam power of theboiler and consequently, the electricity generation.

The arrangement of FIG. 3 utilizes the total heat recovered by coolingof shaft 24 for the electricity generation. Coinciding elements are alsoin this figure identified by the same reference numerals as in FIGS. 1and 2. In addition to the feed water preheater 60 and the condensateheater 62, an evaporator 64 is disposed between the shaft 24 and thefeed water preheater 60 in the circulation gas tubes 30. Water from thefeed water preheater 60 is evaporated in the evaporator by means of theheat of the gas circulation system, whereby 20 bar low-pressure steam isgenerated. The low-pressure steam thereby generated is conveyed, as asteam mixture, together with the exhaust steam from the high-pressuresection 47 of the turbine into the low-pressure section 45 of a2-compartment turbine. High-pressure steam from the boiler is conductedinto the high-pressure section 47 of the turbine. The feed waterrequired by both the boiler and the gas circulation evaporator iscirculated through the condensate heater and the feed water preheater.

The steam turbine in accordance with FIG. 3 is capable of generatingabout 4 MW of electricity if the heat recovered from the gases in theheat recovery system totals 15 MW, whereof the share of the steam boileris 5 MW and the share of the shaft is 10 MW.

All arrangements of FIGS. 1-3 employ a turbine for saturated steam, atthe discharge end of which turbine the allowed steam moisture is on theorder of 20%. The volume and conversion efficiency of the electricitygeneration may also be raised to some extent by means of a superheatingboiler.

According to process conditions, the arrangement of the invention isalso adapted for superheating steam to a suitable degree in the steamboiler.

We claim:
 1. A method of cooling exhaust gases from a molten phasefurnace using a cooling shaft and waste heat boiler, comprising thesteps of:(a) conducting the exhaust gases upwardly via the cooling shaftinto the waste heat boiler; (b) recovering heat from the exhaust gasesin the waste heat boiler in the form of saturated or superheated steam;and (c) positively cooling the gases in the cooling shaft by cooling thecooling shaft with circulating pressurized gas.
 2. Apparatus for coolingexhaust gases from a molten phase furnace, comprising:a molten phasefurnace having a roof and a gas outlet disposed in said roof; a wasteheat boiler; an upwardly extending cooling shaft disposed directly abovesaid furnace and connected to said furnace gas outlet and to said wasteheat boiler, and for conducting exhaust gases from said furnace to saidboiler; a steam turbine connected to said waste heat boiler; heattransfer surfaces provided on said cooling shaft; and an essentiallyclosed gas circulation system for circulating a cooling gas through saidheat transfer surfaces, said gas circulation system comprising apressure compressor for pressurizing the cooling gas, and heat exchangermeans for cooling the cooling gas.
 3. A method as recited in claim 1wherein step (c) is practiced by cooling the cooling shaft withcirculating pressurized uncondensed gas.
 4. A method as recited in claim1 wherein strep (c) is practiced by cooling the cooling shaft withcirculating pressurized air or nitrogen.
 5. A method as recited in claim1 wherein the furnace comprises a melting furnace having a roof and anopening in the furnace roof, and wherein step (a) is practiced byconducting the exhaust gases directly upwardly from the furnace throughan opening in the furnace roof to the cooling shaft.
 6. A method asrecited in claim 1 comprising the further step (d) of recovering heatfrom the pressurized gas used in step (c) utilizing a heat exchanger. 7.A method as recited in claim 1 wherein the waste heat boiler includes apreheater for feed water to the waste heat boiler; and comprising thefurther step (d) of recovering heat from the gas in step (c) utilizingthe preheater for the waste heat boiler feed water.
 8. A method asrecited in claim 1 further utilizing a heater for condensate for thewaste the boiler; and comprising the further strep (d) of recoveringheat from the gas in step (c) using the heater for the waste heat boilercondensate.
 9. A method as recited in claim 1 comprising the furtherstep (d) of recovering heat from the pressurized gas of step (c).
 10. Amethod as recited in claim 9 further utilizing an evaporator for waterfrom the waste heat boiler; and wherein step (d) is practiced utilizingthe evaporator for the water from the waste heat boiler.
 11. A method asrecited in claim 1 wherein in the practice of step (c) the exhaust gasesare cooled to a temperature of between about 600°-900° C.; and whereinstep (b) is practiced to cool the gases to a temperature of betweenabout 330°-380° C.
 12. A method as recited in claim 1 wherein step (c)is practiced by maintaining the pressure of the pressurized gas at about15-25 bar.
 13. A method as recited in claim 1 wherein the furnacecomprises a metal smelting furnace, and wherein the waste heat boiler isdisposed next to the smelting furnace; and comprising the further stepof recovering heat from the exhaust gases of the smelting furnace, theheat being recovered in a first stage by recovering heat from thecirculating gas in step (c), and the heat being recovered in a secondstage as pressurized steam from step (b).
 14. Apparatus as recited inclaim 2 further comprising: at least one tube for conveying cooling gasfrom said cooling shaft to said heat exchanger means; at least one tubefor conveying cooling gas from said heat exchanger means to said coolingshaft; and a circulation fan for circulating cooling gas in saidcirculation system.
 15. Apparatus as recited in claim 2 wherein saidwaste heat boiler extends horizontally and is disposed next to thefurnace, or laterally adjacent said gas outlet of said cooling shaft.16. Apparatus as recited in claim 2 wherein said heat transfer surfacesprovided on said cooling shaft comprise radiation heating surfaces; andwherein said waste heat boiler includes convection heating surfaces forgenerating pressurized steam.
 17. Apparatus as recited in claim 2wherein said heat transfer surfaces provided on said cooling shaftcomprise wall surfaces of said cooling shaft.
 18. Apparatus as recitedin claim 14 wherein said gas circulation system further comprises a heatexchanger means for heating condensate from said steam turbine. 19.Apparatus as recited in claim 2 wherein said gas circulation systemfurther comprises evaporating surfaces for generating steam for saidwaste heat boiler.
 20. Apparatus as recited in claim 2 wherein saidfurnace comprises a metal smelting furnace; and wherein said waste heatboiler extends horizontally and generates superheated steam; and whereinsaid steam turbine comprises a two stage, low pressure and highpressure, steam turbine.
 21. Apparatus as recited in claim 20 whereinsaid gas circulation system further comprises evaporating surfaces forgenerating steam for said low pressure turbine; preheater means forpreheating water for said waste heat boiler; and heating means forheating condensate discharged from said turbine.